A U.S. photojournalist and a South African aid worker were killed in Yemen Saturday during a failed rescue attempt by U.S. commandos. American Luke Somers was shot along with Pierre Korkie by their al-Qaida captors who were tipped off to the rescue attempt by a barking dog. The al-Qaida militants were killed in the resulting firefight. Somers and Korkie were treated on the scene by a U.S. surgical team, but one died on the evacuation aircraft and the other during surgery on a naval ship.

This was the second failed attempt to rescue Somers. In November U.S. operatives located the remote cave where al-Qaida had been hiding him and other hostages. A number of captives were freed, but Somers had been moved.

President Barack Obama said that he authorized the rescue attempt based on information that indicated that Somers’ life was in imminent danger. His captors had released a video statement on Wednesday saying they would kill Somers by Saturday unless their demands were met.

Somers was kidnapped in Yemen in 2013 while outside a supermarket. He appeared this week in a video appealing for help. The video showed a militant from al-Qaida in the Arabian Peninsula (AQAP) threatening to kill him unless their demands, which were not specified, were met. The ultimatum may have been a response to the Nov. 25 rescue attempt.

Korki was captured in May 2013 along with his wife Yolande, who was released in January without a ransom. Korki was expected to be released on Sunday, according to disaster relief organization Gift of the Givers, that claimed to have successfully negotiated his release. The charity has now said that the U.S. rescue attempt had “destroyed everything.”

Today’s early morning operation involved a joint effort between U.S. SEAL Team Six commandos and a small number of counterterrorism troops from Yemeni. They approached the compound by aircraft where the hostages were held, landing several hundred yards away. The compound, guarded by a half dozen or so gunmen, was on alert in anticipation of another possible rescue attempt.

The commandos, wearing night-vision goggles and heavily armed, breached the walled compound under cover of darkness and headed for the building where spy satellites and other American intelligence had pinpointed that the hostages were being held. Then the dog barked and the element of surprise was lost. The commandos saw one of the militants go into the building, but by the time the Americans could get there they had left and Somers and Korkie had been shot and were both seriously wounded.

American officials have said that they were unaware of the arrangements for Korkie’s release. Somers family had recently released a video pleading with captors to release him, and insisting that they had not had any prior knowledge of the previous rescue attempt.

Obama has called Somers’ death a “barbaric murder.” It is not known whether these killings indicate a shift in al-Qaida’s tactics, which, in contrast to the frequent executions carried out by ISIS, has previously been to negotiate ransoms rather than kill hostages.